There are two major types of compression drivers, the first utilizing a dome diaphragm, and the other using an annular flexural diaphragm. The majority of modern annular diaphragms are made of polymer films. The advantage of annular diaphragms is the smaller radial dimensions of the moving part of the diaphragm compared to the dome diaphragms having the same diameter of the moving voice coil. The small radial clamping dimension of the annular diaphragm shifts the mechanical breakup resonances of the diaphragm to higher frequencies where they can be better mechanically damped, since the damping is more efficient at high frequencies in polymer films. Better damping is indicative of the smoother frequency response and lower nonlinear distortion generated by diaphragms' breakups at high frequency.
In a compression driver, the diaphragm is loaded by a compression chamber, which is a thin layer of air separating the diaphragm from a phasing plug. The small radial dimension of the annular diaphragm corresponds to the small radial dimensions of the matching compression chamber, which shifts undesirable air resonances (cross-modes) in the chamber to higher frequencies, sometimes above the audio range in small-format compression drivers. Since the annular diaphragm has two clamping perimeters, inside and outside of the moving part of the diaphragm, the annular diaphragm has a better dynamic stability and it is less prone to the rocking modes compared to a dome diaphragm that has only external clamping.
The volume of air entrapped in the compression chamber is characterized by an acoustical compliance which is proportional to the volume of compression chamber. Acoustical compliance acts as a low-pass filter of the first order and it mitigates the high frequency signal. Therefore, it is desirable to keep the volume of the compression chamber (which depends on the distance between the diaphragm and the phasing plug) low. However, excessively close positioning of the diaphragm to the phasing plug generates distortion due to the nonlinear compression of air in the compression chamber, and may cause rub and buzz or even collision of the diaphragm with the phasing plug. As such, positioning of the diaphragm with respect to the phasing plug is always a compromise.
The area of the entrance to the phasing plug is significantly smaller than the area of the diaphragm. The air paths of the phasing plug are essentially the beginning of the horn which is attached to the compression driver to control directivity (i.e., coverage of sound pressure over a particular listening area) and to increase reproduced sound pressure level over a certain frequency range. The overall acoustical cross-sectional area of the air paths in the phasing plug (there are typically multiple paths) and then of the horn must gradually increase to provide a smooth transition of sound waves to the mouth of the horn. The narrowing of the area would produce undesirable reflections of sound waves back to the entrance of the horn which would interfere with the outgoing sound waves and would produce severe ripples on the sound pressure frequency response.
Compression drivers usually have standard circular exit diameters, typically 1″ for small-format compression drivers, 1.5″, and 2″ for larger format compression drivers. Compression drivers which use an annular diaphragm have an adapter assembly that connects the driver to the horn, where the adapter assembly includes a phasing plug and an outer housing. The phasing plug may include a hub portion or central bullet having an outer surface, and the cylindrical, conical or curved outer housing includes an inner surface. The outer surface and inner surface cooperatively define a waveguide for the propagation of sound waves through the adapter assembly. The output end of the housing may be coupled to the input end of the horn or waveguide by any suitable means, such as via threaded surfaces, with the intention that the waveguide fluidly communicates with the interior of the horn.